There are no standard tests to diagnose Parkinson's disease. Diagnosis is usually based on symptoms, medical history and the results of a clinical examination. Symptoms usually do not develop until 80% of the brain’s dopamine producing neurons are damaged. In the early stages the symptoms are milder and diagnosis is difficult.

Differential diagnosis

Several conditions have symptoms and features similar to Parkinson’s disease. These need to be considered while making a definitive diagnosis of Parkinson’s disease. Some of these include:-

• Benign essential tremor – this is a common form of tremors seen in the elderly. The difference with Parkinson’s disease is that in benign essential tremor, the tremor is worse on movement e.g. while trying to hold a pen or cup. On the other hand in Parkinson’s disease the tremor is seen while at rest.
• Drug or toxin-induced tremor – several drugs and toxins may cause tremor. These include common ones like caffeine, drugs of abuse like amphetamines and medications like selective serotonin reuptake inhibitors (SSRIs), beta-adrenergic blockers, tricyclic antidepressants, lithium, anti-psychotic agents like chlorpromazine and anti-nausea and vomiting agents like prochlorperazine.
• Wilson's disease is a condition with deranged copper metabolism and excretion leading to accumulation of copper in the body and damage to the brain and liver.
• Huntington's disease – a genetic condition with family history of the condition
• Corticobasal degeneration of the brain
• Creutzfeldt-Jakob disease (CJD)
• Pick's disease 
• Multi-infarct dementia and Lewy body dementia 
• Cerebellar tremor that may be caused by a stroke, brainstem tumour, or multiple sclerosis
• Pyschogenic tremor that is caused or increases under direct observation, decreases with distraction.

Diagnosing Parkinson’s

Steps in diagnosis of Parkinson’s disease include:-

• History of family members with the condition and exposure to toxins is obtained.
• The patient is asked to move around or perform a task like write his or her name etc.
• A diagnosis of Parkinson’s disease is likely if there are at least two of the three following symptoms:-
  • Uncontrollable shaking or tremor seen at rest
  • Slowness of movement (bradykinesia)
  • Rigidity and increased muscle tone
• If the symptoms improve rapidly on taking the medication levodopa, Parkinson’s disease is highly likely.
• CT or MRI brain scan is prescribed for patients who fail to respond to therapeutic doses of Levodopa (at least 600 mg/day) for at least 12 weeks. These brain scans reveal rare secondary causes of Parkinsonism like symptoms. The conditions may be supratentorial tumours or a normal pressure hydrocephalus. Functional MRI and CT imaging are useful research tools. These also help in detecting blood flow abnormalities in brain.
• Positron emission tomography (PET) scanning can be ordered to be performed with fluorodopa can localise dopamine deficiency in the basal ganglia.
• Autonomic tests and sphincter electromyography are used to diagnose multiple system atrophy.
• To rule out Wilson’s disease ceruloplasmin levels are measured and tests for the Huntington gene are ordered to exclude Huntington’s disease. Blood tests for syphilis serology are performed to rule out tertiary syphilis.
• Psychometric tests are applied to detect associated depressive disorders and dementia.